1. Field of the Invention
This invention relates to insulating tinsel conductors and, more particularly, to methods of and apparatus for covering a tinsel conductor, which comprises a core having a plurality of tinsel ribbons wrapped helically thereabout, with a polyether polyester thermoplastic copolymer to produce a substantially concentric insulation.
2. Prior Art
Telephone cords which connect, for example, telephone handsets to the telephone base generally comprise a polymeric core having a plurality of tinsel ribbons wrapped helically thereabout. These cords may have either a linear configuration or may be wound in a helical configuration comprising a plurality of convolutions, the latter being referred to as a retractile or spring cord. Telephone cords are well disclosed in the prior art such as, for example, U.S. Pat. No. 3,037,068 issued May 29, 1962 in the name of H. L. Wessel, and in U.S. Pat. Nos. 2,920,351 and 3,024,497 issued on Jan. 12, 1960 and Mar. 13, 1962 respectively in the names of E. C. Hardesty and D. L. Myers, all incorporated by reference hereinto. In the past, tinsel conductors were covered with a nylon knit and then insulated with an extruded polyvinyl chloride (PVC) composition. A plurality of the individually insulated conductors were jacketed with a plasticized PVC composition. See priorly mentioned U.S. Pat. No. 3,037,068.
In a recently introduced modular telephone cord design, miniature type connectors are connected to each end of a line or spring cord to facilitate attachment to telephone instruments. For example, see U.S. Pat. Nos. 3,699,498 and 3,761,869 issued Oct. 17, 1972 and Sept. 25, 1973 respectively in the names of E. C. Hardesty, C. L. Krumreich, A. E. Mulbarger, Jr. and S. W. Walden and in U.S. Pat. No. 3,860,316 issued Jan. 14, 1975 in the name of E. C. Hardesty, all incorporated by reference hereinto.
With the introduction of modularity, it was necessary to use a different cord construction because of a need for a smaller cross-section. In order to reduce the size of the insulated conductor, the knitted nylon covering over the served tinsel was eliminated. The elimination of the protective nylon knit made it necessary to develop a tough insulation material which could function as a high strength barrier to the cutting action of the tinsel ribbon, as an electrical insulation over the tinsel conductor, and as a primary component to achieve resiliency in a retractile telephone cord. A plasticized nylon insulation was used as a replacement for the knitted nylon covering.
The use of nylon in insulating tinsel conductors has not been altogether satisfactory. Occasionally, portions of one or more of the tinsel ribbons protrude outwardly and cause protruberances in the slow crystallizing nylon insulation. The nylon-insulated conductors must be rewound and passed through a die to eliminate the protruberances. The plasticized nylon also has a tendency to creep under load thereby diminishing somewhat the effectiveness of the strain relief system of the modular plugs disclosed in aforementioned U.S. Pat. Nos. 3,699,498, 3,761,869, and 3,860,316.
In order to overcome these difficulties, cords destined to be used in the modular program and generally having an oval shaped configuration are now manufactured with the insulation over the tinsel conductors being a polyether polyester thermoplastic composition as disclosed and claimed in U.S. Pat. No. 4,090,763 in the names of W. I. Congdon, J. J. Mottine and W. C. Vesperman. That application discloses and claims that the insulation cover comprises a polyether polyester composition obtained by reacting 1, 4 butane diol terephalate with terephalate esters of polytetramythalene glycol. A plasticized polyvinyl chloride jacket is formed over the plurality of the individually insulated tinsel conductors. A material such as that disclosed and claimed in the above-identified Congdon et al application is available commercially from E. I. duPont Company under the trade name HYTREL .RTM.7246 polyester elastomer.
In attempting to manufacture tinsel conductors having an insulation which overcomes the difficulties described hereinbefore, methods and apparatus are provided for applying a reliable continuously concentric insulation and one which avoids the problem of tinsel protrusions into the insulation which necessitate the rewind operation.
Typically, insulation is tubed over tinsel conductors as disclosed, for example, in U.S. Pat. No. 3,553,042 issued on Jan. 5, 1971 in the name of E. R. Cocco and incorporated by reference hereinto. The tubing provides that the insulation material is spaced from the tinsel conductors to provide for relative movement therebetween thus adding to the flexibility and long life of the telephone cord.
In a typical tubing operation, the downstream end of a core tube in an extruder crosshead extends at least to the vicinity of a die opening and in some constructions beyond the opening.
Generally in die designs, the included angle between a converging frustoconical portion of the inner wall of the die adjacent to and converging from the die opening is on an order of magnitude of from 30.degree. to approximately 60.degree.. This included angle is often referred to as the angle of attack or angle of approach.
Information relating to typical angles of attack may be found, for example, on page 117 of Wire and Cable Coaters' Handbook as published by the E. I. duPont Company in 1968, and in Plastics Extrusion Technology and Theory authorized by Gerhard Schenkel and published by the American Elsevier Publishing Co. in 1966 and by Karl Hanser in Germany in 1963. See also pages 198 to 241 of Plastics Extrusion Technology, by A. L. Griff, published by Rinehold Book Corp. and an article entitled "Crosshead Tooling for Jacket Extrusion" by Joe B. Moss on pages 25-28 of the April, 1967 issue of the Western Electric Engineer. Also, see duPont brochure entitled HYTREL.RTM. Polyester Elastomer Rheology and Handling and, Wire Extrusion Techniques, pages 5-6 by D. C. Hank as published by the B. F. Goodrich Chemical Company and U.S. Pat. No. 3,382,535 issued May 14, 1968 in the name of A. G. Ferrari, all incorporated by reference hereinto.
The prior art also shows techniques for controlling the engagement of the tubed plastic extrudate with the core being enclosed. In U.S. Pat. No. 3,211,818 strands are held out of contact with an extruded tubular housing until it becomes sufficiently form-sustaining such as, for example, by cooling, so as not to be forced out of shape or damaged when the strands are allowed to come into contact therewith.
In U.S. Pat. No. 3,227,786 compressed air is introduced into the core tube during the extrusion of the jacket to control shrinkage of the jacket to prevent any compression of the cable core and rearrangement of the strands comprising the core to avoid any increase in capacitance unbalance.
In U.S. Pat. No. 2,291,670 a wire or other filamentous article is coated with a crystalline polymer in oriented form by extruding the crystalline but unoriented polymer at a temperature, preferably above its softening point, after which the wire is drawn through a mandrel and out of a nozzle at a greater linear velocity than that at which the crystalline polymer is being extruded, supercooled and then stretched to provide molecular orientation. Because of the mandrel in the nozzle, the polymer coating emerges therefrom distributed around the wire with an annular space between the wire and the coating.
U.S. Pat. Nos. 2,218,459 and 3,112,828 show extrusion dies for extruding metals in which a beveled area adjacent the die opening has an approach angle on the order of 120.degree. to 140.degree..
U.S. Pat. No. 3,255,621 shows a die having a lubrication die insert adjacent the die exit port. U.S. Pat. No. 3,267,748 shows an extrusion process for forming a solid cross sectional shape form a billet extruded by a piston. U.S. Pat. No. 3,400,428 shows extruding very viscous thermoplastic materials by using a screw conveyor with a rotatable body being mounted between the conveyor and a die opening.